The present invention relates generally to the field of gravure printing. More particularly, it concerns methods for determining various parameters of gravure cylinders including the cell volume, the cell width, cell wall width, the channel width or the space, the cell surface area, etc. to optimize the production of high quality engraved gravure cylinders.
Gravure printing is done on presses that use cylinders that have text and images engraved on their surfaces. Consequently, the printing plates are engraved to create cells or depressions in the areas containing the text and images. To print using these cylinders, the cells are filled with ink. A doctor blade is then used to remove excess ink from the nonprinting areas or cell walls. The cells are engraved into a gravure cylinder by an engraving head of an engraver or engraving machine such as a Helio-Klischograph manufactured by Dr. Ing. Rudolf Hell GmbH. The engraving head includes a diamond stylus cutting tool.
Prior to engraving a gravure cylinder, each engraving head of the engraver is calibrated. Calibration is performed by engraving selected test cuts on the gravure cylinder. Each test cut is composed of a collection of preferably identical cells. Typically, at least two test cuts are made before an image is engraved onto the gravure cylinder. Normally, one test cut of highlight cells is engraved at the light end of the image which corresponds to a stylus digital value (dv), for example, of 161. A second test cut of shadow cells is engraved at the dark end, or in the shadows of the image, which corresponds, for example, to a dv of 1. Tests cuts are sometimes made in the midtone areas which normally correspond to a dv of 81.
Finally, a tone reproduction curve, in the form of an 8-bit (256 level) look-up table, maps the image data to the engrave data. This table allows for fine tone-reproduction adjustments throughout the entire printing range for each printing color.
The process of calibration requires measurement of certain cylinder parameters. An operator usually measures the morphological parameters of a single cell out of the test cut with an optical microscope. This general procedure is performed for a highlight cell and a shadow cell in the respective test cuts for each engraving head. The operator usually knows from experience a target highlight cell width (wh), a target shadow cell width (ws), and a third parameter. The third parameter depends upon the type of shadow cells being engraved. If the shadow cells are connected, i.e., have connecting channels between the cells in the circumferential direction, the third parameter is channel width (wc). If the cells are discrete, i.e., have spaces between the cells in the circumferential direction, the third parameter is the space (S). The space S is related to the length of a cell by the equation:
dl=S+length
where dl is the stylus period, also known as the circumferential spacing of the cells;
S is the space; and
length is the length of a cell in the circumferential direction.
Another parameter that defines a gravure cylinder is the maximum cell width (dq). Usually, cells are engraved in an offset fashion in the axial direction. In other words, a first circumferential line of cells is engraved around the cylinder. A second circumferential line of cells is then engraved around the cylinder, but the second line of cells is offset from the first line of cells such that, in the case of cells connected by channels, the horizontal center of the cells in the first line are at the channels of the second line, as illustrated in FIG. 1. Therefore, the maximum cell width (dq) is the measurement from the middle of the channel of the first line of cells to the middle of the channel of the third line of cells. The second line of cells begins a distance dq/2 away from the first line of cells, but that beginning point is offset by dl/2 in the circumferential direction. The raster and angle (or cell shape) uniquely define dl and dq for a given cylinder. For example, a 70 raster with a compressed cell shape has a dl=172.5 microns and a dqxe2x88x92230 microns. A table of raster values and their corresponding dl and dq values for various cell shapes is provided in FIG. 2. These values are unique to the Helio-Klischograph engraving machine. There are a variety of cell shapes including compressed, normal, elongated, coarse, and fine. These are illustrated in FIG. 3. Another gravure cylinder parameter is the cell wall width. The cell wall width is the measurement of the axial spacing between cells. In the case of cells connected by channels, the cell wall width 18 is constant, as illustrated in FIG. 1. In the case of discrete cells, the cell wall width 58 varies, as illustrated in FIG. 4. The cell wall is required in order to support the doctor blade that removes the excess ink from the cylinder. Another parameter that affects cell size is the stylus angle; however, this angle is usually not changed.
Target values for cylinder engraving parameters are fine tuned by trial and error. These adjustments are motivated by observations made during press runs. For example, a press crew might notice that flesh tones are tending to look too red. The engraving department might try to fix this by making a small reduction in the target width of the cells on the magenta cylinder. The degree of adjustment is determined by trial and error, but experienced engravers will probably be able to make such an adjustment with relatively few trials. Conversely, a customer might complain that a printed product looks grainy. The engraver may choose to fix this by engraving with a finer screen, i.e., engraving at a higher raster. This move to a higher raster, will require significant changes to all the engraving parameters in the cylinders for every color. The correct adjustments to make in this case are outside the expertise of most cylinder engraving departments because raster changes are a much less frequent occurrence. These significant engraving parameter changes can lead to significant and costly trial and error procedures. Thus, there is a need for an improved method of determining the change in cell parameters when significant cylinder adjustments are necessary, such as a change in raster, cell shape and/or stylus angle.
It is an object of the present invention to provide a method of determining engraving parameters for a gravure cylinder at a desired raster, cell shape and stylus angle given an estimate of the engraving parameters.
It is another object of the present invention to provide a method of determining a set of engraving parameters for a gravure cylinder at a new ink cut.
It is a further object of the present invention to provide a method of determining the amount of ink solution that a given gravure cylinder will use in making a specified number of impressions.
It is another object of the present invention to provide a method of determining, prior to printing, the total cost associated with making a specified number of impressions.
It is still another object of the present invention to provide a method of determining the optimal cell geometry for a gravure cylinder.
These and other objects of the invention are provided by a method of calculating engraving parameters for a gravure cylinder at a desired raster, cell shape and stylus angle given an estimate of the engraving parameters. The method includes inputting a set of initial parameters including an initial raster, an initial cell shape, an initial stylus angle, an initial highlight width, an initial shadow width, an initial channel width and an initial space. The method proceeds by calculating an initial value of a cell volume from the initial parameters. Next, a set of new parameters are inputted including a desired raster, an estimate of a desired highlight width, an estimate of a desired shadow width, an estimate of a desired channel width and an estimate of a desired space. Then, a new value of the cell volume is calculated from the new parameters. Using the volume calculation, the method can be used to: calculate a set of engraving parameters for a gravure cylinder at a new ink cut, calculate the amount of ink solution that a given gravure cylinder will use in making a specified number of impressions, determine the total cost associated with making a specified number of impressions and determine the optical cell geometry for a gravure cylinder.